1. Field of the Invention
The present invention is directed generally to a method and apparatus for measuring a fluid and, more particularly, for measuring a fluid by maintaining first and second heaters at a constant temperature and measuring relative power consumption of the heaters.
2. Description of the Background
Flow sensors and meters are used to determine the fluid flow of a known fluid. One type of prior art device measures fluid flow by volumetric means, wherein the volume of fluid passing a point in a known time is used to calculate flow rate. However, because the volume of a given mass of a fluid changes with fluid pressure and temperature, volumetric measurements may not be accurate. For example, an increase in the internal pressure of the fluid system will cause the volume occupied by a given number of fluid molecules to decrease. That decrease in volume will cause an error in readings of mass flow rate by a volumetric method.
Other prior art devices, known as heat transfer mass flow rate sensors, measure fluid flow by mass, not volume, so as to be unaffected by changes in fluid temperature and pressure. Such sensors include means for heating the fluid as it passes through a sensor tube and means of detecting the rate at which the heat applied to the fluid is carried away by the fluid flowing through the sensor tube.
Different designs have been utilized to heat a sensor tube of heat transfer mass flow rate sensors. Early designs inductively heated the sensor tube with constant power. Later designs employed a resistor temperature heating device powered by a constant voltage or a constant current. Other designs heated the sensor tube with a high resistance winding to which was applied a constant power.
Prior art heat transfer mass flow rate sensors have a number of drawbacks. For example, they are slow to respond to sudden fluid flow rate changes. This is because increased fluid flow also increases the amount of heat carried away from the sensor tube, cooling down the sensor tube. Thus, the temperature of the sensor tubes of prior art sensors will be a function of the flow rate through them. Typically, a passive temperature change can be modeled as a decaying exponential, and due to the mass of the sensor tube the time constant for prior art sensors is on the order of a second. Normally, it takes several time constants for temperature shifts to come to equilibrium. Therefore, the prior art sensors will tend to be slow to respond to sudden changes of fluid flow. Moreover, many prior art sensors will have a non-linearity problem because the reduction in sensor tube temperature with increasing mass flow will result in a decreasing sensitivity to further increases in mass flow. Thus, many prior art sensors cannot measure more than about 10 cm.sup.3 /minute of fluid at atmospheric pressure without becoming too nonlinear to properly calibrate.
Prior art sensors also suffer from thermal drift problems because they measure the temperature of the sensor tube before and after the heater device, such as with thermocouples, resistor temperature devices (RTD), or thermistors. The output from those sensors assumes that the differential temperature between the inlet and outlet temperature detectors is proportional to the mass flow rate. However, this is only strictly true when the sensor operates at the temperature at which it was calibrated.
Some prior art heat transfer mass flow rate sensors provide some type of first order correction to this sensing error by changing the drive power with temperature or by incorporating a temperature-sensitive gain stage in the output amplifiers. Other sensors includes sensor tube heaters that are designed to operate at a constant temperature regardless of the magnitude of fluid flow. Such sensors may not have the slow response to flow rate changes discussed above, but as the ambient air temperature increases the differential temperature and signal output decreases.
Other prior art sensors are designed so that the ambient temperature around the sensor is maintained at a constant temperature that is above the maximum expected ambient operating temperature, while also heating the sensor tube to another higher temperature. Such sensors may satisfactorily treat both response time and temperature difference variation problems, but are mechanically very complex and require significant power to raise the ambient temperature surrounding the sensor tube to a temperature above the maximum expected operating temperature. Such constant temperature sensors also suffer an additional deficiency. Typically, a mass flow sensor measures the flow rate of a small percentage of a much larger fluid flow. The measured portion of the flow is split off from the main fluid flow by a laminar flow element bypass unit. Constant temperature sensors produce a temperature dependent flow split error between the sensor and the flow bypass. The flow split error occurs because the fluid in the sensor tube is maintained at a constant temperature that gives it a constant viscosity, while variations in the ambient temperature produce viscosity variations in the flow bypass fluid. Thus, constant temperature sensors may treat one source of error, but introduce another source of error.
Certain early heat transfer mass flow rate sensors utilize the same RTDs to heat the fluid flow and to measure the fluid flow temperature. All such sensors use the voltage across the bridge as the sensor output, which introduces a non-linearity in the output versus flow.
Accordingly, a need exists for a cost effective apparatus and method to accurately measure fluid flow under a wide variety of operating conditions.